Injection Molding of Polyarylene Sulfide Compositions

ABSTRACT

A method for injection molding a thermoplastic composition that contains a polyarylene sulfide and a boron-containing nucleating agent is provided. By selectively controlling certain aspects of the polyarylene sulfide and nucleating agent, as well as the particular manner in which they are combined, the crystallization properties of the resulting thermoplastic composition can be significantly improved. This allows the “cooling time” during a molding cycle to be substantially reduced while still achieving the same degree of crystallization. The cooling time can be represented by the “normalized cooling ratio”, which is determined by dividing the total cooling time by the average thickness of the molded part.

RELATED APPLICATIONS

This application claims filing benefit of U.S. Provisional Patent Application Ser. No. 61/576,505 filed on Dec. 16, 2011, which is incorporated herein in its entirety.

BACKGROUND OF THE INVENTION

Polyphenylene sulfide (“PPS”) is a high performance polymer that can withstand high thermal, chemical, and mechanical stresses. Due to its relatively slow crystallization rate, however, injection molding of parts from polyphenylene sulfide can be challenging. For example, to achieve the desired degree of crystallization, molding is generally conducted at a high mold temperature (˜130° C. or more) and for a relatively long cycle time. Unfortunately, high mold temperatures typically dictate the need for expensive and corrosive heating mediums (e.g., oil). Attempts to address the problems noted above have generally involved the inclusion of various additives in the polymer composition to help improve its crystallization properties. To date, however, such attempts have not been fully satisfactory. In fact, the problems have become even more pronounced as various industries (e.g., electronic, automotive, etc.) are now demanding injection molded parts with very small dimensional tolerances. In these applications, the polymer must have good flow properties so that it can quickly and uniformly fill the small spaces of the mold cavity. It has been found, however, that conventional polyphenylene sulfides that manage to meet the requisite high flow requirement tend to require a long cooling cycle, which can be a very costly and time consuming step.

As such, a need exists for a suitable method for injection molding polyarylene sulfide with a relatively short cycle time while still achieving good mechanical properties.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a method for injection molding a thermoplastic composition is disclosed. The method comprises injecting a thermoplastic composition into a mold cavity, wherein the thermoplastic composition comprises a polyarylene sulfide and a boron-containing nucleating agent; subjecting the thermoplastic composition to a cooling cycle to form a molded part having a certain average thickness, wherein the cooling cycle has a normalized cooling ratio of from about 0.2 to about 8 seconds per millimeter, as determined by dividing the total time of the cooling cycle by the thickness of the molded part; and ejecting the molded part from the mold cavity.

In accordance with another embodiment of the present invention, a molded part is disclosed that is formed from a thermoplastic composition that comprises a polyarylene sulfide and boron-containing nucleating agent. For example, in one embodiment a centrifugal pump for circulating a coolant through an automotive engine is disclosed. The pump comprises a pump impeller that is configured to flow the coolant radially outward into a volute chamber; and a housing that encloses the pump impeller and volute chamber, wherein at least a portion of the pump impeller, the housing, or both comprise a molded part formed from the thermoplastic composition. In another embodiment, an electronic device is disclosed that comprises a molded part such as a portion of the housing or a cooling fan that is formed from the thermoplastic composition.

Other features and aspects of the present invention are set forth in greater detail below.

BRIEF DESCRIPTION OF THE FIGURES

A full and enabling disclosure of the present invention, including the best mode thereof to one skilled in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures, in which:

FIG. 1 is a cross-sectional view of one embodiment of an injection mold apparatus that may be employed in the present invention; and

FIG. 2 illustrates a water pump that may be formed in accordance with one embodiment of the present invention.

FIG. 3 is a perspective view of an electronic device that can be formed in accordance with one embodiment of the present invention; and

FIG. 4 is a perspective view of the electronic device of FIG. 3, shown in closed configuration.

DETAILED DESCRIPTION

It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present invention.

Generally speaking, the present invention is directed to a method for injection molding a thermoplastic composition that contains a polyarylene sulfide and a boron-containing nucleating agent. By selectively controlling certain aspects of the polyarylene sulfide and nucleating agent, as well as the particular manner in which they are combined, the present inventors have discovered that the crystallization properties of the resulting thermoplastic composition can be significantly improved. Among other things, this allows the “cooling time” during a molding cycle to be substantially reduced while still achieving the same degree of crystallization. The cooling time can be represented by the “normalized cooling ratio”, which is determined by dividing the total cooling time by the average thickness of the molded part. As a result of the present invention, for example, the normalized cooling ratio may range from about 0.2 to about 8 seconds per millimeter, in some embodiments from about 0.5 to about 6 seconds per millimeter, and in some embodiments, from about 1 to about 5 seconds per millimeter. The total cooling time can be determined from the point when the composition is injected into the mold cavity to the point that it reaches an ejection temperature at which it can be safely ejected. Exemplary cooling times may range, for instance, from about 1 to about 60 seconds, in some embodiments from about 5 to about 40 seconds, and in some embodiments, from about 10 to about 35 seconds. Likewise, exemplary average thicknesses for the molded parts may be about 25 millimeters or less, in some embodiments from about 0.5 to about 15 millimeters, and in some embodiments, from about 1 millimeter to about 10 millimeters.

In addition to minimizing the required cooling time for a molding cycle, the method and composition of the present invention can also allow parts to be molded at lower temperatures while still achieving the same degree of crystallization. For example, the mold temperature (e.g., temperature of a surface of the mold) may be from about 50° C. to about 120° C., in some embodiments from about 60° C. to about 110° C., and in some embodiments, from about 70° C. to about 90° C. In addition to minimizing the energy requirements for the molding operation, such low mold temperatures may be accomplished using cooling mediums that are less corrosive and expensive than some conventional techniques. For example, liquid water may be employed as a cooling medium. Further, the use of low mold temperatures can also decrease the production of “flash” normally associated with high temperature molding operations. For example, the length of any flash (also known as burrs) created during a molding operation may be about 0.17 millimeters or less, in some embodiments about 0.14 millimeters or less, and in some embodiments, about 0.13 millimeters or less.

Various embodiments of the present invention will now be described in greater detail below.

I. Thermoplastic Composition

A. Polyarylene Sulfide

As noted above, the thermoplastic composition contains at least one polyarylene sulfide, which is generally able to withstand relatively high temperatures without melting. Although the actual amount may vary depending on desired application, polyarylene sulfide(s) typically constitute from about 30 wt. % to about 95 wt. %, in some embodiments from about 35 wt. % to about 90 wt. %, and in some embodiments, from about 40 wt. % to about 80 wt. % of the thermoplastic composition. The polyarylene sulfide(s) generally have repeating units of the formula:

—[(Ar¹)_(n)—X]_(m)—[(Ar²)_(i)—Y]_(j)—[(Ar³)_(k)—Z]_(l)—[(Ar⁴)_(o)—W]_(p)

wherein,

Ar¹, Ar², Ar³, and Ar⁴ are independently arylene units of 6 to 18 carbon atoms;

W, X, Y, and Z are independently bivalent linking groups selected from —SO₂—, —S—, —SO—, —CO—, —O—, —C(O)O— or alkylene or alkylidene groups of 1 to 6 carbon atoms, wherein at least one of the linking groups is —S—; and

n, m, i, j, k, l, o, and p are independently 0, 1, 2, 3, or 4, subject to the proviso that their sum total is not less than 2.

The arylene units Ar¹, Ar², Ar³, and Ar⁴ may be selectively substituted or unsubstituted. Advantageous arylene units are phenylene, biphenylene, naphthylene, anthracene and phenanthrene. The polyarylene sulfide typically includes more than about 30 mol %, more than about 50 mol %, or more than about 70 mol % arylene sulfide (—S—) units. For example, the polyarylene sulfide may include at least 85 mol % sulfide linkages attached directly to two aromatic rings. In one particular embodiment, the polyarylene sulfide is a polyphenylene sulfide, defined herein as containing the phenylene sulfide structure —(C₆H₄—S)_(n)—(wherein n is an integer of 1 or more) as a component thereof.

Synthesis techniques that may be used in making a polyarylene sulfide are generally known in the art. By way of example, a process for producing a polyarylene sulfide can include reacting a material that provides a hydrosulfide ion (e.g., an alkali metal sulfide) with a dihaloaromatic compound in an organic amide solvent. The alkali metal sulfide can be, for example, lithium sulfide, sodium sulfide, potassium sulfide, rubidium sulfide, cesium sulfide or a mixture thereof. When the alkali metal sulfide is a hydrate or an aqueous mixture, the alkali metal sulfide can be processed according to a dehydrating operation in advance of the polymerization reaction. An alkali metal sulfide can also be generated in situ. In addition, a small amount of an alkali metal hydroxide can be included in the reaction to remove or react impurities (e.g., to change such impurities to harmless materials) such as an alkali metal polysulfide or an alkali metal thiosulfate, which may be present in a very small amount with the alkali metal sulfide.

The dihaloaromatic compound can be, without limitation, an o-dihalobenzene, m-dihalobenzene, p-dihalobenzene, dihalotoluene, dihalonaphthalene, methoxy-dihalobenzene, dihalobiphenyl, dihalobenzoic acid, dihalodiphenyl ether, dihalodiphenyl sulfone, dihalodiphenyl sulfoxide or dihalodiphenyl ketone. Dihaloaromatic compounds may be used either singly or in any combination thereof. Specific exemplary dihaloaromatic compounds can include, without limitation, p-dichlorobenzene; m-dichlorobenzene; o-dichlorobenzene; 2,5-dichlorotoluene; 1,4-dibromobenzene; 1,4-dichloronaphthalene; 1-methoxy-2,5-dichlorobenzene; 4,4′-dichlorobiphenyl; 3,5-dichlorobenzoic acid; 4,4′-dichlorodiphenyl ether; 4,4′-dichlorodiphenylsulfone; 4,4′-dichlorodiphenylsulfoxide; and 4,4′-dichlorodiphenyl ketone. The halogen atom can be fluorine, chlorine, bromine or iodine, and two halogen atoms in the same dihalo-aromatic compound may be the same or different from each other. In one embodiment, o-dichlorobenzene, m-dichlorobenzene, p-dichlorobenzene or a mixture of two or more compounds thereof is used as the dihalo-aromatic compound. As is known in the art, it is also possible to use a monohalo compound (not necessarily an aromatic compound) in combination with the dihaloaromatic compound in order to form end groups of the polyarylene sulfide or to regulate the polymerization reaction and/or the molecular weight of the polyarylene sulfide.

The polyarylene sulfide(s) may be homopolymers or copolymers. For instance, selective combination of dihaloaromatic compounds can result in a polyarylene sulfide copolymer containing not less than two different units. For instance, when p-dichlorobenzene is used in combination with m-dichlorobenzene or 4,4′-dichlorodiphenylsulfone, a polyarylene sulfide copolymer can be formed containing segments having the structure of formula:

and segments having the structure of formula:

or segments having the structure of formula:

In another embodiment, a polyarylene sulfide copolymer may be formed that includes a first segment with a number-average molar mass Mn of from 1000 to 20,000 g/mol. The first segment may include first units that have been derived from structures of the formula:

where the radicals R¹ and R², independently of one another, are a hydrogen, fluorine, chlorine or bromine atom or a branched or unbranched alkyl or alkoxy radical having from 1 to 6 carbon atoms; and/or second units that are derived from structures of the formula:

The first unit may be p-hydroxybenzoic acid or one of its derivatives, and the second unit may be composed of 2-hydroxynaphthalene-6-carboxylic acid. The second segment may be derived from a polyarylene sulfide structure of the formula:

—[Ar—S]_(q)—

where Ar is an aromatic radical, or more than one condensed aromatic radical, and q is a number from 2 to 100, in particular from 5 to 20. The radical Ar may be a phenylene or naphthylene radical. In one embodiment, the second segment may be derived from poly(m-thiophenylene), from poly(o-thiophenylene), or from poly(p-thiophenylene).

The polyarylene sulfide(s) may be linear, semi-linear, branched or crosslinked. Linear polyarylene sulfides typically contain 80 mol % or more of the repeating unit —(Ar—S)—. Such linear polymers may also include a small amount of a branching unit or a cross-linking unit, but the amount of branching or cross-linking units is typically less than about 1 mol % of the total monomer units of the polyarylene sulfide. A linear polyarylene sulfide polymer may be a random copolymer or a block copolymer containing the above-mentioned repeating unit. Semi-linear polyarylene sulfides may likewise have a cross-linking structure or a branched structure introduced into the polymer a small amount of one or more monomers having three or more reactive functional groups. By way of example, monomer components used in forming a semi-linear polyarylene sulfide can include an amount of polyhaloaromatic compounds having two or more halogen substituents per molecule which can be utilized in preparing branched polymers. Such monomers can be represented by the formula R′X_(n), where each X is selected from chlorine, bromine, and iodine, n is an integer of 3 to 6, and R′ is a polyvalent aromatic radical of valence n which can have up to about 4 methyl substituents, the total number of carbon atoms in R′ being within the range of 6 to about 16. Examples of some polyhaloaromatic compounds having more than two halogens substituted per molecule that can be employed in forming a semi-linear polyarylene sulfide include 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, 1,3-dichloro-5-bromobenzene, 1,2,4-triiodobenzene, 1,2,3,5-tetrabromobenzene, hexachlorobenzene, 1,3,5-trichloro-2,4,6-trimethylbenzene, 2,2′,4,4′-tetrachlorobiphenyl, 2,2′,5,5′-tetra-iodobiphenyl, 2,2′,6,6′-tetrabromo-3,3′,5,5′-tetramethylbiphenyl, 1,2,3,4-tetrachloronaphthalene, 1,2,4-tribromo-6-methylnaphthalene, etc., and mixtures thereof.

Regardless of the particular structure, the number average molecular weight of the polyarylene sulfide is typically about 15,000 g/mol or more, and in some embodiments, about 30,000 g/mol or more. In certain cases, a small amount of chlorine may be employed during formation of the polyarylene sulfide. Nevertheless, the polyarylene sulfide will still have a low chlorine content, such as about 1000 ppm or less, in some embodiments about 900 ppm or less, in some embodiments from about 1 to about 800 ppm, and in some embodiments, from about 2 to about 700 ppm. In certain embodiments, however, the polyarylene sulfide is generally free of chlorine or other halogens.

B. Boron-Containing Nucleating Agent

A boron-nucleating agent is also employed in the thermoplastic composition of the present invention to enhance the crystallization properties of the composition. Such nucleating agents typically constitute from about 0.01 wt. % to about 6 wt. %, in some embodiments from about 0.05 wt. % to about 3 wt. %, and in some embodiments, from about 0.1 wt. % to about 2 wt. % of the thermoplastic composition. Suitable boron-containing nucleating agents may include, for instance, boron nitride, sodium tetraborate, potassium tetraborate, calcium tetraborate, etc., as well as mixtures thereof. Boron nitride (BN) has been found to be particularly beneficial. Boron nitride exists in a variety of different crystalline forms (e.g., h-BN—hexagonal, c-BN—cubic or spharlerite, and w-BN—wurtzite), any of which can generally be employed in the present invention. The hexagonal crystalline form is particularly suitable due to its stability and softness.

C. Other Additives

In addition to the nucleating agent and polyarylene sulfide, the thermoplastic composition may also contain a variety of other different components to help improve its overall properties. One suitable additive that may be employed to improve the mechanical properties of the composition is an impact modifier. Examples of suitable impact modifiers may include, for instance, polyepoxides, polyurethanes, polybutadiene, acrylonitrile-butadiene-styrene, polysiloxanes etc., as well as mixtures thereof. In one particular embodiment, a polyepoxide modifier is employed that contains at least two oxirane rings per molecule. The polyepoxide may be a linear or branched, homopolymer or copolymer (e.g., random, graft, block, etc.) containing terminal epoxy groups, skeletal oxirane units, and/or pendent epoxy groups. The monomers employed to form such polyepoxides may vary. In one particular embodiment, for example, the polyepoxide modifier contains at least one epoxy-functional (meth)acrylic monomeric component. The term “(meth)acrylic” includes acrylic and methacrylic monomers, as well as salts or esters thereof, such as acrylate and methacrylate monomers. Suitable epoxy-functional (meth)acrylic monomers may include, but are not limited to, those containing 1,2-epoxy groups, such as glycidyl acrylate and glycidyl methacrylate. Other suitable epoxy-functional monomers include allyl glycidyl ether, glycidyl ethacrylate, and glycidyl itoconate.

If desired, additional monomers may also be employed in the polyepoxide to help achieve the desired melt viscosity. Such monomers may vary and include, for example, ester monomers, (meth)acrylic monomers, olefin monomers, amide monomers, etc. In one particular embodiment, for example, the polyepoxide modifier includes at least one linear or branched α-olefin monomer, such as those having from 2 to 20 carbon atoms and preferably from 2 to 8 carbon atoms. Specific examples include ethylene, propylene, 1-butene; 3-methyl-1-butene; 3,3-dimethyl-1-butene, 1-pentene; 1-pentene with one or more methyl, ethyl or propyl substituents; 1-hexene with one or more methyl, ethyl or propyl substituents; 1-heptene with one or more methyl, ethyl or propyl substituents; 1-octene with one or more methyl, ethyl or propyl substituents; 1-nonene with one or more methyl, ethyl or propyl substituents; ethyl, methyl or dimethyl-substituted 1-decene; 1-dodecene; and styrene. Particularly desired α-olefin comonomers are ethylene and propylene. In one particularly desirable embodiment of the present invention, the polyepoxide modifier is a copolymer formed from an epoxy-functional (meth)acrylic monomeric component and α-olefin monomeric component. For example, the polyepoxide modifier may be poly(ethylene-co-glycidyl methacrylate). One specific example of a suitable polyepoxide modifier that may be used in the present invention is commercially available from Arkema under the name Lotader® AX8840. Lotader® AX8950 has a melt flow rate of 5 g/10 min and has a glycidyl methacrylate monomer content of 8 wt. %.

Still another suitable additive that may be employed to improve the mechanical properties of the thermoplastic composition is an organosilane coupling agent. The coupling agent may, for example, be any alkoxysilane coupling agent as is known in the art, such as vinlyalkoxysilanes, epoxyalkoxysilanes, aminoalkoxysilanes, mercaptoalkoxysilanes, and combinations thereof. Aminoalkoxysilane compounds typically have the formula: R⁵—Si—(R⁶)₃, wherein R⁵ is selected from the group consisting of an amino group such as NH₂; an aminoalkyl of from about 1 to about 10 carbon atoms, or from about 2 to about 5 carbon atoms, such as aminomethyl, aminoethyl, aminopropyl, aminobutyl, and so forth; an alkene of from about 2 to about 10 carbon atoms, or from about 2 to about 5 carbon atoms, such as ethylene, propylene, butylene, and so forth; and an alkyne of from about 2 to about 10 carbon atoms, or from about 2 to about 5 carbon atoms, such as ethyne, propyne, butyne and so forth; and wherein R⁶ is an alkoxy group of from about 1 to about 10 atoms, or from about 2 to about 5 carbon atoms, such as methoxy, ethoxy, propoxy, and so forth. In one embodiment, R⁵ is selected from the group consisting of aminomethyl, aminoethyl, aminopropyl, ethylene, ethyne, propylene and propyne, and R⁶ is selected from the group consisting of methoxy groups, ethoxy groups, and propoxy groups. In another embodiment, R⁵ is selected from the group consisting of an alkene of from about 2 to about 10 carbon atoms such as ethylene, propylene, butylene, and so forth, and an alkyne of from about 2 to about 10 carbon atoms such as ethyne, propyne, butyne and so forth, and R⁶ is an alkoxy group of from about 1 to about 10 atoms, such as methoxy group, ethoxy group, propoxy group, and so forth. A combination of various aminosilanes may also be included in the mixture.

Some representative examples of aminosilane coupling agents that may be included in the mixture include aminopropyl triethoxysilane, aminoethyl triethoxysilane, aminopropyl trimethoxysilane, aminoethyl trimethoxysilane, ethylene trimethoxysilane, ethylene triethoxysilane, ethyne trimethoxysilane, ethyne triethoxysilane, aminoethylaminopropyltrimethoxysilane, 3-aminopropyl triethoxysilane, 3-aminopropyl trimethoxysilane, 3-aminopropyl methyl dimethoxysilane or 3-aminopropyl methyl diethoxysilane, N-(2-aminoethyl)-3-aminopropyl trimethoxysilane, N-methyl-3-aminopropyl trimethoxysilane, N-phenyl-3-aminopropyl trimethoxysilane, bis(3-aminopropyl)tetramethoxysilane, bis(3-aminopropyl)tetraethoxy disiloxane, and combinations thereof. The amino silane may also be an aminoalkoxysilane, such as γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane, γ-aminopropylmethyldimethoxysilane, γ-aminopropylmethyldiethoxysilane, N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane, N-phenyl-γ-aminopropyltrimethoxysilane, γ-diallylaminopropyltrimethoxysilane and γ-diallylaminopropyltrimethoxysilane. One suitable amino silane is 3-aminopropyltriethoxysilane which is available from Degussa, Sigma Chemical Company, and Aldrich Chemical Company.

Fillers may also be employed in the thermoplastic composition to help achieve the desired properties and/or color. When employed, such mineral fillers typically constitute from about 5 wt. % to about 60 wt. %, in some embodiments from about 10 wt. % to about 50 wt. %, and in some embodiments, from about 15 wt. % to about 45 wt. % of the thermoplastic composition. Clay minerals may be particularly suitable for use in the present invention. Examples of such clay minerals include, for instance, talc (Mg₃Si₄O₁₀(OH)₂), halloysite (Al₂Si₂O₅(OH)₄), kaolinite (Al₂Si₂O₅(OH)₄), illite ((K,H₃O)(Al,Mg,Fe)₂(Si,Al)₄O₁₀[(OH)₂,(H₂O)]), montmorillonite (Na,Ca)_(0.33)(Al,Mg)₂Si₄O₁₀(OH)₂.nH₂O), vermiculite ((MgFe,Al)₃(Al,Si)₄O₁₀(OH)₂.4H₂O), palygorskite ((Mg,Al)₂Si₄O₁₀(OH).4(H₂O)), pyrophyllite (Al₂Si₄O₁₀(OH)₂), etc., as well as combinations thereof. In lieu of, or in addition to, clay minerals, still other mineral fillers may also be employed. For example, other suitable silicate fillers may also be employed, such as calcium silicate, aluminum silicate, mica, diatomaceous earth, wollastonite, and so forth. Mica, for instance, may be a particularly suitable mineral for use in the present invention. There are several chemically distinct mica species with considerable variance in geologic occurrence, but all have essentially the same crystal structure. As used herein, the term “mica” is meant to generically include any of these species, such as muscovite (KAl₂(AlSi₃)O₁₀(OH)₂), biotite (K(Mg,Fe)₃(AlSi₃)O₁₀(OH)₂), phlogopite (KMg₃(AlSi₃)O₁₀(OH)₂), lepidolite (K(Li,Al)₂₋₃(AISi₃)O₁₀(OH)₂), glauconite (K,Na)(Al,Mg,Fe)₂(Si,Al)₄O₁₀(OH)₂), etc., as well as combinations thereof.

Fibrous fillers may also be employed in the thermoplastic composition. When employed, such fibrous fillers typically constitute from about 5 wt. % to about 60 wt. %, in some embodiments from about 10 wt. % to about 50 wt. %, and in some embodiments, from about 15 wt. % to about 45 wt. % of the thermoplastic composition. The fibrous fillers may include one or more fiber types including, without limitation, polymer fibers, glass fibers, carbon fibers, metal fibers, and so forth, or a combination of fiber types. In one embodiment, the fibers may be chopped glass fibers or glass fiber rovings (tows). Fiber diameters can vary depending upon the particular fiber used and are available in either chopped or continuous form. The fibers, for instance, can have a diameter of less than about 100 μm, such as less than about 50 μm. For instance, the fibers can be chopped or continuous fibers and can have a fiber diameter of from about 5 μm to about 50 μm, such as from about 5 μm to about 15 μm.

Lubricants may also be employed in the thermoplastic composition that are capable of withstanding the processing conditions of poly(arylene sulfide) (typically from about 290° C. to about 320° C.) without substantial decomposition. Exemplary of such lubricants include fatty acids esters, the salts thereof, esters, fatty acid amides, organic phosphate esters, and hydrocarbon waxes of the type commonly used as lubricants in the processing of engineering plastic materials, including mixtures thereof. Suitable fatty acids typically have a backbone carbon chain of from about 12 to about 60 carbon atoms, such as myristic acid, palmitic acid, stearic acid, arachic acid, montanic acid, octadecinic acid, parinric acid, and so forth. Suitable esters include fatty acid esters, fatty alcohol esters, wax esters, glycerol esters, glycol esters and complex esters. Fatty acid amides include fatty primary amides, fatty secondary amides, methylene and ethylene bisamides and alkanolamides such as, for example, palmitic acid amide, stearic acid amide, oleic acid amide, N,N′-ethylenebisstearamide and so forth. Also suitable are the metal salts of fatty acids such as calcium stearate, zinc stearate, magnesium stearate, and so forth; hydrocarbon waxes, including paraffin waxes, polyolefin and oxidized polyolefin waxes, and microcrystalline waxes. Particularly suitable lubricants are acids, salts, or amides of stearic acid, such as pentaerythritol tetrastearate, calcium stearate, or N,N′-ethylenebisstearamide. When employed, the lubricant(s) typically constitute from about 0.05 wt. % to about 1.5 wt. %, and in some embodiments, from about 0.1 wt. % to about 0.5 wt. % of the thermoplastic composition.

Still another additive that may be employed in the thermoplastic composition is a disulfide compound. Without wishing to be bound by any particular theory, the disulfide compound can undergo a polymer scission reaction with a polyarylene sulfide during melt processing that even further lowers the overall melt viscosity of the composition. When employed, disulfide compounds typically constitute from about 0.01 wt. % to about 3 wt. %, in some embodiments from about 0.02 wt. % to about 1 wt. %, and in some embodiments, from about 0.05 to about 0.5 wt. % of the composition. The ratio of the amount of the polyarylene sulfide to the amount of the disulfide compound may likewise be from about 1000:1 to about 10:1, from about 500:1 to about 20:1, or from about 400:1 to about 30:1. Suitable disulfide compounds are typically those having the following formula:

R³—S—S—R⁴

wherein R³ and R⁴ may be the same or different and are hydrocarbon groups that independently include from 1 to about 20 carbons. For instance, R³ and R⁴ may be an alkyl, cycloalkyl, aryl, or heterocyclic group. In certain embodiments, R³ and R⁴ are generally nonreactive functionalities, such as phenyl, naphthyl, ethyl, methyl, propyl, etc. Examples of such compounds include diphenyl disulfide, naphthyl disulfide, dimethyl disulfide, diethyl disulfide, and dipropyl disulfide. R³ and R⁴ may also include reactive functionality at terminal end(s) of the disulfide compound. For example, at least one of R³ and R⁴ may include a terminal carboxyl group, hydroxyl group, a substituted or non-substituted amino group, a nitro group, or the like. Examples of compounds may include, without limitation, 2,2′-diaminodiphenyl disulfide, 3,3′-diaminodiphenyl disulfide, 4,4′-diaminodiphenyl disulfide, dibenzyl disulfide, dithiosalicyclic acid, dithioglycolic acid, α,α′-dithiodilactic acid, β,β′-dithiodilactic acid, 3,3′-dithiodipyridine, 4,4′dithiomorpholine, 2,2′-dithiobis(benzothiazole), 2,2′-dithiobis(benzimidazole), 2,2′-dithiobis(benzoxazole) and 2-(4′-morpholinodithio)benzothiazole.

Still other additives that can be included in the composition may include, for instance, antimicrobials, pigments, antioxidants, stabilizers, surfactants, waxes, flow promoters, solid solvents, and other materials added to enhance properties and processability.

The manner in which the boron-containing nucleating agent, polyarylene sulfide, and other optional additives are combined may vary as is known in the art. For instance, the materials may be supplied either simultaneously or in sequence to a melt processing device that dispersively blends the materials. Batch and/or continuous melt processing techniques may be employed. For example, a mixer/kneader, Banbury mixer, Farrel continuous mixer, single-screw extruder, twin-screw extruder, roll mill, etc., may be utilized to blend and melt process the materials. One particularly suitable melt processing device is a co-rotating, twin-screw extruder (e.g., Leistritz co-rotating fully intermeshing twin screw extruder). Such extruders may include feeding and venting ports and provide high intensity distributive and dispersive mixing. For example, the polyarylene sulfide and nucleating agent may be fed to the same or different feeding ports of a twin-screw extruder and melt blended to form a substantially homogeneous melted mixture. Melt blending may occur under high shear/pressure and heat to ensure sufficient dispersion. For example, melt processing may occur at a temperature of from about 50° C. to about 500° C., and in some embodiments, from about 100° C. to about 250° C. Likewise, the apparent shear rate during melt processing may range from about 100 seconds⁻¹ to about 10,000 seconds⁻¹, and in some embodiments, from about 500 seconds⁻¹ to about 1,500 seconds⁻¹. Of course, other variables, such as the residence time during melt processing, which is inversely proportional to throughput rate, may also be controlled to achieve the desired degree of homogeneity.

Besides melt blending, other techniques may also be employed to combine the nucleating agent and the polyarylene sulfide. For example, the nucleating agent may be supplied during one or more stages of the polymerization of the polyarylene sulfide, such as to the polymerization apparatus. Although it may be introduced at any time, it is typically desired to apply the nucleating agent before polymerization has been initiated, and typically in conjunction with the precursor monomers for the polyarylene sulfide. The reaction mixture is generally heated to an elevated temperature within the polymerization reactor vessel to initiate melt polymerization of the reactants.

Regardless of the manner in which they are combined together, the degree and rate of crystallization may be significantly enhanced by the nucleation system of the present invention. For example, the crystallization potential of the thermoplastic composition (prior to molding) may be about 52% or more, in some embodiments about 55% or more, in some embodiments about 58% or more, and in some embodiments, from about 60% to about 95%. The crystallization potential may be determined by subtracting the latent heat of crystallization (ΔH_(c)) from the latent heat of fusion (ΔH_(f)), dividing this difference by the latent heat of fusion, and then multiplying by 100. The latent heat of fusion (ΔH_(f)) and latent heat of crystallization (ΔH_(c)) may be determined by Differential Scanning calorimetry (“DSC”) as is well known in the art and in accordance with ISO Standard 10350. The latent heat of crystallization may, for example, be about 15 Joules per gram (“J/g”) or less, in some embodiments about 12 J/g or less, and in some embodiments, from about 1 to about 10 J/g. The latent heat of fusion may likewise be about 15 Joules per gram (“J/g”) or more, in some embodiments about 18 J/g or more, and in some embodiments, from about 20 to about 28 J/g.

In addition, the thermoplastic composition may also crystallize at a lower temperature than would otherwise occur absent the presence of the nucleating agent. For example, the crystallization temperature (prior to molding) of the thermoplastic composition may about 250° C. or less, in some embodiments from about 100° C. to about 245° C., and in some embodiments, from about 150° C. to about 240° C. The melting temperature of the thermoplastic composition may also range from about 250° C. to about 320° C., and in some embodiments, from about 265° C. to about 300° C. The melting and crystallization temperatures may be determined as is well known in the art using differential scanning calorimetry in accordance with ISO Test No. 11357. Even at such melting temperatures, the ratio of the deflection temperature under load (“DTUL”), a measure of short term heat resistance, to the melting temperature may still remain relatively high. For example, the ratio may range from about 0.65 to about 1.00, in some embodiments from about 0.70 to about 0.99, and in some embodiments, from about 0.80 to about 0.98. The specific DTUL values may, for instance, range from about 230° C. to about 300° C., in some embodiments from about 240° C. to about 290° C., and in some embodiments, from about 250° C. to about 280° C. Such high DTUL values can, among other things, allow the use of high speed processes often employed during the manufacture of components having a small dimensional tolerance.

The present inventors have also discovered that the thermoplastic composition may possess a relatively low melt viscosity, which allows it to readily flow into the mold cavity during production of the part. For instance, the composition may have a melt viscosity of about 20 poise or less, in some embodiments about 15 poise or less, and in some embodiments, from about 0.1 to about 10 poise, as determined by a capillary rheometer at a temperature of 316° C. and shear rate of 1200 seconds⁻¹. Among other things, these viscosity properties can allow the composition to be readily injection molded into parts having very small dimensions without producing excessive amounts of flash.

II. Injection Molding

The method of the present invention includes the injection of the thermoplastic composition into a mold cavity where it is cooled until reaching the desired ejection temperature. As discussed above, the unique properties of the thermoplastic composition of the present invention can allow the cooling time and/or mold temperature of a molding cycle to be substantially reduced while still achieving the same degree of crystallization. In addition to improving the properties of the cooling cycle, other aspects of the molding operation may also be enhanced. For example, as is known in the art, injection can occur in two main phases—i.e., an injection phase and holding phase. During the injection phase, the mold cavity is completely filled with the molten thermoplastic composition. The holding phase is initiated after completion of the injection phase in which the holding pressure is controlled to pack additional material into the cavity and compensate for volumetric shrinkage that occurs during cooling. After the shot has built, it can then be cooled. In addition to reducing the cooling time as discussed above, the improved properties of the thermoplastic composition may also allow for a lower holding time, which includes the time required to pack additional material into the cavity and the time at which this material is held at a certain pressure. Once cooling is complete, the molding cycle is completed when the mold opens and the part is ejected, such as with the assistance of ejector pins within the mold.

Any suitable injection molding equipment may generally be employed in the present invention. Referring to FIG. 1, for example, one embodiment of an injection molding apparatus or tool 10 that may be employed in the present invention is shown. In this embodiment, the apparatus 10 includes a first mold base 12 and a second mold base 14, which together define an article or component-defining mold cavity 16. The molding apparatus 10 also includes a resin flow path that extends from an outer exterior surface 20 of the first mold half 12 through a sprue 22 to a mold cavity 16. The resin flow path may also include a runner and a gate, both of which are not shown for purposes of simplicity. The thermoplastic composition may be supplied to the resin flow path using a variety of techniques. For example, the thermoplastic composition may be supplied (e.g., in the form of pellets) to a feed hopper attached to an extruder barrel that contains a rotating screw (not shown). As the screw rotates, the pellets are moved forward and undergo pressure and friction, which generates heat to melt the pellets. Additional heat may also be supplied to the composition by a heating medium that is communication with the extruder barrel. One or more ejector pins 24 may also be employed that are slidably secured within the second mold half 14 to define the mold cavity 16 in the closed position of the apparatus 10. The ejector pins 24 operate in a well-known fashion to remove a molded part from the cavity 16 in the open position of the molding apparatus 10.

A cooling mechanism may also be provided to solidify the resin within the mold cavity. In FIG. 1, for instance, the mold bases 12 and 14 each include one or more cooling lines 18 through which a cooling medium flows to impart the desired mold temperature to the surface of the mold bases for solidifying the molten material. As noted above, the present inventors have found that the improved crystallization properties of the thermoplastic composition can allow it to be molded at lower mold temperatures than previously thought possible. In addition to minimizing the energy requirements for the molding operation, such lower mold temperatures may also be achieved using mechanisms that are less corrosive and expensive than some conventional techniques. For example, liquid water may be employed as the cooling medium and may be heated to a temperature within the ranges noted above.

As a result of the injection molding technique employed in the present invention, it has been discovered that the thermoplastic composition can be readily formed into parts having a wide range of different parts. The parts may be in the form of a substrate having an average thickness of about 25 millimeters or less, in some embodiments from about 0.5 to about 15 millimeters, and in some embodiments, from about 1 millimeter to about 10 millimeters. Alternatively, the part may simply possess certain features (e.g., walls, ridges, etc.) within the average thickness ranges noted above.

Regardless of the particular size, the present inventors have discovered that excellent mechanical properties can be achieved even when relatively short cooling times are employed. For instance, an injection molded part may exhibit a tensile strength of from about 100 to about 500 MPa, in some embodiments from about 120 to about 400 MPa, and in some embodiments, from about 190 to about 350 MPa. The part may also exhibit a tensile break strain of about 0.5% or more, in some embodiments from about 0.6% to about 10%, and in some embodiments, from about 0.8% to about 3.5%; and/or a tensile modulus of from about 5,000 MPa to about 25,000 MPa, in some embodiments from about 8,000 MPa to about 22,000 MPa, and in some embodiments, from about 10,000 MPa to about 20,000 MPa. The tensile properties may be determined in accordance with ISO Test No. 527 (technically equivalent to ASTM D638) at 23° C. The parts may also exhibit a flexural strength of from about 20 to about 500 MPa, in some embodiments from about 50 to about 400 MPa, and in some embodiments, from about 100 to about 350 MPa; a flexural break strain of about 0.5% or more, in some embodiments from about 0.6% to about 10%, and in some embodiments, from about 0.8% to about 3.5%; and/or a flexural modulus of from about 5,000 MPa to about 25,000 MPa, in some embodiments from about 8,000 MPa to about 22,000 MPa, and in some embodiments, from about 10,000 MPa to about 20,000 MPa. The flexural properties may be determined in accordance with ISO Test No. 178 (technically equivalent to ASTM D790) at 23° C. The part may also possess a high impact strength, such as an Izod notched impact strength greater than about 4 kJ/m², in some embodiments from about 5 to about 40 kJ/m², and in some embodiments, from about 6 to about 30 kJ/m², measured at 23° C. according to ISO Test No. 180) (technically equivalent to ASTM D256, Method A).

With such excellent mechanical properties, the resulting injection molded parts may be employed in a wide variety of different components. One particular component that may incorporate an injection molded part of the present invention is a liquid pump (e.g., water pump). The liquid pump may be a direct lift pump, positive displacement pump (e.g., rotary, reciprocating, or linear), rotodynamic pump (e.g., centrifugal), gravity pump, etc. Rotodynamic pumps, in which energy is continuously imparted to the pumped fluid by a rotating impeller, propeller, or rotor, are particularly suitable. In a centrifugal pump, for instance, fluid enters a pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute chamber, from which it exits into the downstream piping. Such pumps are often used in automotive applications to move a coolant through the engine. Due to the high temperatures associated with automotive engines, the injection molded composition of the present invention is particularly well suited for use in the centrifugal pumps of such automotive cooling systems. In certain embodiments, for example, all or a portion (e.g., blades) of the water impeller may be formed from the injection molded composition of the present invention. Centrifugal pumps also generally include a housing that encloses certain components of the pump and protects them from heat, corrosion, etc. In some embodiments, some or all of the housing may be formed from the injection molded composition of the present invention.

Referring to FIG. 2, one particular example of a centrifugal pump is shown that can employ the injection molded composition of the present invention. In the illustrated embodiment, the pump contains a rotary shaft 201 supported on a housing 203 via a bearing 202. A pump impeller 204, which may contain the injection molded composition of the present invention, is rigidly fixed at an end of the rotary shaft 201. A pulley hub 205 is also rigidly fixed on the base end portion of the rotary shaft 201. Between the bearing 202 and the pump impeller 204, a mechanical seal 206 is formed that is constituted by a stationary member 206 a fixed on the side of the housing 203 and a rotary member 206 b fixedly engaged with the rotary shaft 201. The pump may also include a housing 207, which can contain the injection molded composition of the present invention. The housing 207 may be affixed to the pump housing 203 (e.g., with fastening bolts) so that a volute chamber 208 is defined therebetween. While not illustrated, a suction portion and a discharge port may also be provided within the housing 207.

Of course, the injection molded composition is not limited to the formation of water pumps or portions thereof, and it may be utilized in forming all manner of components as may be incorporated in a fluid handling system including pipes and sections of pipes, flanges, valves, valve seats, seals, sensor housings, thermostats, thermostat housings, diverters, linings, propellers, and so forth. The composition may also employed in forming components that function in a fluid environment, such as consumer products that encounter high temperature fluids, e.g., heated beverage containers. Still further, the composition may be employed in completely different environments, such as an electronic component. Examples of electronic components that may employ such a molded part include, for instance, cellular telephones, laptop computers, small portable computers (e.g., ultraportable computers, netbook computers, and tablet computers), wrist-watch devices, pendant devices, headphone and earpiece devices, media players with wireless communications capabilities, handheld computers (also sometimes called personal digital assistants), remote controllers, global positioning system (GPS) devices, handheld gaming devices, battery covers, speakers, camera modules, integrated circuits (e.g., SIM cards), etc.

Wireless electronic devices, however, are particularly suitable. Examples of suitable wireless electronic devices may include a desktop computer or other computer equipment, a portable electronic device, such as a laptop computer or small portable computer of the type that is sometimes referred to as “ultraportables.” In one suitable arrangement, the portable electronic device may be a handheld electronic device. Examples of portable and handheld electronic devices may include cellular telephones, media players with wireless communications capabilities, handheld computers (also sometimes called personal digital assistants), remote controls, global positioning system (“GPS”) devices, and handheld gaming devices. The device may also be a hybrid device that combines the functionality of multiple conventional devices. Examples of hybrid devices include a cellular telephone that includes media player functionality, a gaming device that includes a wireless communications capability, a cellular telephone that includes game and email functions, and a handheld device that receives email, supports mobile telephone calls, has music player functionality and supports web browsing.

Referring to FIGS. 3-4, one particular embodiment of an electronic device 100 is shown as a portable computer. The electronic device 100 includes a display member 103, such as a liquid crystal diode (LCD) display, an organic light emitting diode (OLED) display, a plasma display, or any other suitable display. In the illustrated embodiment, the device is in the form of a laptop computer and so the display member 103 is rotatably coupled to a base member 106. It should be understood, however, that the base member 106 is optional and can be removed in other embodiments, such as when device is in the form of a tablet portable computer. Regardless, in the embodiment shown in FIGS. 3-4, the display member 103 and the base member 106 each contain a housing 86 and 88, respectively, for protecting and/or supporting one or more components of the electronic device 100. The housing 86 may, for example, support a display screen 120 and the base member 106 may include cavities and interfaces for various user interface components (e.g., keyboard, mouse, and connections to other peripheral devices). Although the thermoplastic composition of the present invention may generally be employed to form any portion of the electronic device 100, for example for forming the cooling fan, it is typically employed to form all or a portion of the housing 86 and/or 88. When the device is a tablet portable computer, for example, the housing 88 may be absent and the thermoplastic composition may be used to form all or a portion of the housing 86. Regardless, due to the unique properties achieved by the present invention, the housing(s) or a feature of the housing(s) may be molded to have a very small wall thickness, such as within the ranges noted above.

Although not expressly shown, the device 100 may also contain circuitry as is known in the art, such as storage, processing circuitry, and input-output components. Wireless transceiver circuitry in circuitry may be used to transmit and receive radio-frequency (RF) signals. Communications paths such as coaxial communications paths and microstrip communications paths may be used to convey radio-frequency signals between transceiver circuitry and antenna structures. A communications path may be used to convey signals between the antenna structure and circuitry. The communications path may be, for example, a coaxial cable that is connected between an RF transceiver (sometimes called a radio) and a multiband antenna.

The present invention may be better understood with reference to the following examples.

Test Methods

Melt Viscosity:

The melt viscosity is determined as scanning shear rate viscosity and determined in accordance with ISO Test No. 11443 (technically equivalent to ASTM D3835) at a shear rate of 1200 s⁻¹ and at a temperature of 316° C. using a Dynisco 7001 capillary rheometer. The rheometer orifice (die) had a diameter of 1 mm, a length of 20 mm, an L/D ratio of 20.1, and an entrance angle of 180°. The diameter of the barrel was 9.55 mm+0.005 mm and the length of the rod was 233.4 mm.

Thermal Properties:

The thermal properties are determined by differential scanning calorimetry (“DSC”) in accordance with ISO Test No. 11357. Under the DSC procedure, samples are heated and cooled at 20° C. per minute as stated in ISO Standard 10350 using DSC measurements conducted on a TA Q100 Instrument. For both pellet and mold samples, the heating and cooling program is a 2-cycle test that begins with an equilibration of the chamber to 25° C., followed by a first heating period at a heating rate of 20° C. per minute to a temperature of 320° C., followed by equilibration of the sample at 320° C. for 1 minutes, followed by a first cooling period at a cooling rate of 20° C. per minute to a temperature of 50° C., followed by equilibration of the sample at 50° C. for 1 minute, and then a second heating period at a heating rate of 20° C. per minute to a temperature of 320° C. The results are evaluated using a TA software program, which identifies and quantifies the melting temperature, the endothermic and exothermic peaks, and the areas under the peaks on the DSC plots. The areas under the peaks on the DSC plots are determined in terms of joules per gram of sample (J/g). For example, the heat of fusion of a resin or mold sample is determined by integrating the area of the endothermic peak. The area values are determined by converting the areas under the DSC plots (e.g., the area of the endotherm) into the units of joules per gram (J/g) using computer software. The exothermic heat of crystallization is determined during the first cooling cycle and the second heating cycle. The percent crystallization potential may also be calculated as follows:

% crystallization potential=100*(A−B)/A

wherein,

A is the sum of endothermic peak areas (e.g., 1st heat of fusion); and

B is the sum of exothermic peak areas (e.g., pre-crystallization heat of fusion).

Tensile Modulus, Tensile Stress, and Tensile Elongation:

Tensile properties are tested according to ISO Test No. 527 (technically equivalent to ASTM D638). Modulus and strength measurements are made on the same test strip sample having a length of 80 mm, thickness of 10 mm, and width of 4 mm. The testing temperature is 23° C., and the testing speeds are 1 or 5 mm/min.

Flexural Modulus, Flexural Stress, and Flexural Strain:

Flexural properties are tested according to ISO Test No. 178 (technically equivalent to ASTM D790). This test is performed on a 64 mm support span. Tests are run on the center portions of uncut ISO 3167 multi-purpose bars. The testing temperature is 23° C. and the testing speed is 2 mm/min.

Izod Notched Impact Strength:

Notched Izod properties are tested according to ISO Test No. 180 (technically equivalent to ASTM 0256, Method A). This test is run using a Type A notch. Specimens are cut from the center of a multi-purpose bar using a single tooth milling machine. The testing temperature is 23° C.

Deflection Under Load Temperature (“DTUL”):

The deflection under load temperature is determined in accordance with ISO Test No. 75-2 (technically equivalent to ASTM D648-07). A test strip sample having a length of 80 mm, thickness of 10 mm, and width of 4 mm is subjected to an edgewise three-point bending test in which the specified load (maximum outer fibers stress) is 1.8 MPa. The specimen is lowered into a silicone oil bath where the temperature is raised at 2° C. per minute until it deflects 0.25 mm (0.32 mm for ISO Test No. 75-2).

Flash:

To determine flash, the sample is initially dried at 135° C. for 3 to 4 hours. The sample is then injection molded into a dual tab flash mold using the following conditions: melt temperature of 321° C., injection time of 1.5 seconds, injection pressure of 30,000 psi, hold time and pressure of 10 seconds at 1,000 psi, and screw retraction time of 20 seconds. More particularly, the sample is injected so that 0.5 inches of one tab is filled in 1.5 seconds with resin and 0.75 inches of the other tab remains unfilled. After cooling, the flash of the parts is measured with a Proscan instrument coupled with Image Pro Plus software.

Example 1

The components listed in Table 1 below are mixed in a Werner Pfleiderer ZSK 25 co-rotating intermeshing twin-screw extruder with an 18 mm diameter.

TABLE 1 Sample Components Glass FORTRON ® Boron Fibers 0205 Nitride Glycolube P Aminosilane (4 mm) Sample PPS (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) 1 59.3 — 0.3 0.4 40.0 2 59.1 0.2 0.3 0.4 40.0

The thermal properties of pellets formed from the samples are determined, the results of which are set forth below in Table 2.

TABLE 2 Thermal Properties Pre- Pre- 1^(st) 2^(nd) Re- Cryst Cryst Heat, 1^(st) Heat, 2^(nd) Re- Cryst Melt Heat of Melt Heat of Melt Heat of Cryst Heat of Cryst MV Temp Fusion Temp Fusion Temp Fusion Temp Fusion Potential Sample (poise) (° C.) (J/g) (° C.) (J/g) (° C.) (J/g) (° C.) (J/g) (%) 1 2648 126.1 11.1 280.9 22.7 277.4 22.2 214.3 23.1 51.1 2 2590 124.8 8.2 281.0 21.3 280.8 21.4 236.5 23.6 61.4

As indicated above, the addition of the boron nitride nucleating agent increased the crystallization potential of the composition.

Pellets formed from the Samples 1-2 are also molded into T-bars (thickness of 3 millimeters) on a Mannesmann Demag D100 NCl II injection molding machine at various different cooling cycle times and/or mold temperatures. The mechanical properties are tested, the results of which are set forth below in Table 3.

TABLE 3 Mechanical Properties Cooling Normalized Tensile Mold Cycle Cooling stress Temp. Time Ratio (5 mm/min) DTUL Flash Sample (° C.) (s) (s/mm) (MPa) (° C.) (mm) 1 135 25.0 8.3 195 272 0.50 90 25.0 8.3 188 270 0.14 90 12.5 4.2 187 270 — 60 25.0 8.3 193 268 0.07 65 12.5 4.2 188 268 — 2 135 25.0 8.3 194 273 0.65 90 25.0 8.3 194 273 0.19 90 12.5 4.2 194 273 — 60 25.0 8.3 191 272 0.10 65 12.5 4.2 197 272 —

As indicated above, Sample 2 (containing boron nitride) generally exhibited better mechanical properties when the cooling cycle time and/or mold temperature is reduced. For example, at a cooling time of 12.5 seconds (Normalized Cooling Ratio of 4.2 s/mm) and mold temperature of 65° C., Sample 2 achieved a tensile stress of 197 MPa, while Sample 1 was only able to achieve a tensile stress of 188 MPa.

These and other modifications and variations of the present invention may be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present invention. In addition, it should be understood that aspects of the various embodiments may be interchanged both in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention so further described in such appended claims. 

What is claimed is:
 1. A method for injection molding a thermoplastic composition, the method comprising: injecting a thermoplastic composition into a mold cavity, wherein the thermoplastic composition comprises a polyarylene sulfide and a boron-containing nucleating agent; subjecting the thermoplastic composition to a cooling cycle to form a molded part having a certain average thickness, wherein the cooling cycle has a normalized cooling ratio of from about 0.2 to about 8 seconds per millimeter, as determined by dividing the total time of the cooling cycle by the thickness of the molded part; and ejecting the molded part from the mold cavity.
 2. The method of claim 1, wherein the normalized cooling ratio is from about 1 to about 5 seconds per millimeter.
 3. The method of claim 1, wherein the total time of the cooling cycle is from about 5 to about 40 seconds, and wherein the average thickness of the molded part is from about 0.5 to about 15 millimeters.
 4. The method of claim 1, wherein the mold temperature is from about 50° C. to about 120° C.
 5. The method of claim 1, wherein the mold temperature is from about 70° C. to about 90° C.
 6. The method of claim 1, wherein the water is used as a cooling medium during the cooling cycle.
 7. The method of claim 1, wherein polyarylene sulfides constitute from about 30 wt. % to about 95 wt. % of the composition and boron-containing nucleating agents constitute from about 0.01 wt. % to about 6 wt. % of the composition.
 8. The method of claim 1, wherein the boron-containing nucleating agent includes boron nitride.
 9. The method of claim 1, wherein the composition further comprises an impact modifier, mineral filler, fibrous filler, organosilane coupling agent, lubricant, disulfide, or a combination thereof.
 10. The method of claim 1, wherein the composition has a crystallization potential of about 52% or more, as determined by differential scanning calorimetry in accordance with ISO
 10350. 11. The method of claim 1, wherein the molded part has a tensile strength of from about 100 to about 500 MPa, as determined in accordance with ISO Test No. 527 at 23° C.
 12. The method of claim 1, wherein the molded part has a tensile strength of from about 120 to about 400 MPa, as determined in accordance with ISO Test No. 527 at 23° C.
 13. An injection molded part formed according to the method of claim
 1. 14. A liquid pump comprising the injection molded part of claim
 13. 15. The liquid pump of claim 14, wherein the pump is a direct lift pump, positive displacement pump, rotodynamic pump, or gravity pump.
 16. The liquid pump of claim 14, wherein the pump is a centrifugal pump for circulating a coolant through an automotive engine.
 17. An electronic component comprising the injection molded part of claim
 13. 18. The electronic component of claim 17, wherein the component is a cellular telephone, laptop computer, small portable computer, wrist-watch device, pendant device, headphone or earpiece device, media player with wireless communication capabilities, handheld computer, remote controller, global positioning system device, handheld gaming device, battery cover, speaker, camera module, or integrated circuit.
 19. A centrifugal pump for circulating a coolant through an automotive engine, wherein the pump comprises: a pump impeller that is configured to flow the coolant radially outward into a volute chamber; and a housing that encloses the pump impeller and volute chamber, wherein at least a portion of the pump impeller, the housing, or both comprise a molded part formed from a thermoplastic composition that comprises a polyarylene sulfide and boron-containing nucleating agent. 